The present application is based upon Japanese Patent Application No. HEI 3 310288 filed Sep. 13, 1991, a disclosure of which is incorporated herein by reference.
The present invention relates to a zoom lens system that is suitable for use with compact cameras which have a small constraint on back focus. More particularly, the present invention relates to a zoom lens system that features a high zoom ratio of 2.5 and more.
Various types of zoom lens systems have heretofore been known for use with compact cameras. Zoom lenses consisting of three lens groups or more with a zoom ratio exceeding 2 are categorized as follows:
(i) Four-group zoom lens systems comprising a positive first lens group, a negative second lens group, a stop diaphragm disposed behind the second lens group, as well as a positive third lens group and a negative fourth lens group, all lens groups being moved independently of each other towards the object (this type includes a system where some of the four lens groups are moved in unison). Examples of such zoom lens systems are disclosed in Japanese Patent Laid-Open Publications No. SHO 63-43115, No. SHO 63-159818 and No. SHO63-157120. PA1 (ii) Three-group zoom lens systems comprising a positive first lens group, a positive second lens group (which includes a stop diaphragm and comprises a negative sub-group 2a disposed closer to the object than the diaphragm and a positive sub-group 2b disposed closer to the image plane than the diaphragm and which is equivalent to the second and third lens groups of the four-group zoom lens system (i)), and a negative third lens group (which is equivalent to the fourth group of the four-group zoom lens system (i)), all lens groups being moved independently of each other towards the object. Examples of this zoom lens system are disclosed in Japanese Patent Laid-Open Publications No. SHO 63-153511 and No. SHO 63-161423. PA1 (iii) Three-group zoom lens systems comprising a positive first lens group, a positive second lens group (as in the system (ii), this positive lens group comprises, in order from the object side, a negative sub-group 2a and a positive sub-group 2b but no stop diaphragm is included), a stop diaphragm disposed behind the second lens group, as well as a negative third lens group with all lens groups being moved towards the object (see, for example, commonly assigned Japanese Patent Laid-Open Publication No. HEI 2-73211). PA1 the radius of curvature of the surface on the object side of the first lens element in the first lens group; and PA1 r12: PA1 the radius of curvature of the surface on the image plane side of the first lens element in the first lens group.
(iv) A practically four-group zoom lens system that is the same as the system (iii) except that the sub-groups 2a and 2b are moved independently of each other (see Example 3 in the specification of commonly assigned Japanese Patent Application No. SHO 63-225294, supra).
The above-described conventional zoom lens systems have their own problems. In the system (i), all of the four lens groups have to be moved independently of each other, so a large number of cams must be used in a restricted small space, causing difficulty in designing an effective mechanical composition.
In the systems (i) and (ii), the stop diaphragm is disposed between the sub-groups 2a and 2b (in the case of the four-group type, the stop diaphragm is disposed between the second and third lens groups); however, the shutter block also serving as a stop diaphragm must be disposed between the two lens groups which are subject to substantial deterioration in performance due to any manufacturing errors, so the overall lens arrangement will become complex, making it difficult to insure high mechanical precision. In addition, the high optical sensitivity of either system has made it difficult to achieve stabilization in optical performance.
Further, all systems (i), (ii), (iii) and (iv) have one problem in common; that is, the overall compactness of those systems is insufficient for using them with a compact camera and this is because the negative first lens element has a concave surface of such a large curvature on the image plane side that the overall length of the first lens group is increased, which is unfavorable for the purpose of reducing the lens diameter and the overall length of the entire lens system.
The present invention has been accomplished under these circumstances and has as an object providing a zoom lens system for use with a compact camera that is particularly improved in the first lens group to achieve a high zoom ratio of 2.5 and more. Another object of the present invention is to provide a zoom lens system suitable for use with a compact camera that successfully achieves reduction in the lens diameter and the overall lens length.